Downhole tractors are often employed to drive a well tool through a horizontal or highly deviated well at an oilfield. In this manner, the tool may be positioned at a well location of interest in spite of the non-vertical nature of such wells. Different configurations of downhole tractors may be employed for use in such a well. For example, a passive tractor having tractor arms in the form of separate adjacent sondes with immobilizing traction elements thereon may be employed. With such a tractor, the sondes may alternatingly be immobilized against a borehole casing at the well wall and advanced in an inchworm-like fashion through the well. Alternatively, an active or continuous movement tractor employing tractor arms with driven traction elements thereon may be employed. Such driven traction elements may include wheels, cams, pads, tracks, or chains. With this type of tractor, the driven traction elements may be in continuous movement at the borehole casing interface, thus driving the tractor through the well.
Regardless of the tractor configuration chosen, the tractor along with several thousand pounds of equipment may be pulled thousands of feet into the well for performance of an operation at the well location of interest. In order to achieve this degree of tractoring, radial forces are imparted from the tractor toward the well wall through the noted traction elements. In this manner, the tractor may avoid slippage and be advanced through the well.
The effectiveness of the described radial forces in avoiding slippage and ensuring tractor advancement may depend on the centralized positioning of the tractor within the well. For example, as noted above, the well may be lined with a borehole casing of a circumferential nature. Thus, the tractor may be positioned in a centralized manner relative to the casing in order to ensure that a proper interface of the tractor and the casing is maintained. That is, with a properly centralized tractor, balanced interfacing between the traction elements and the borehole casing may be ensured thereby optimizing the amount of driving friction between the casing and the elements.
In addition to optimizing the traction element-borehole casing interface for improved driving friction, centralization may also provide other tractoring advantages. Furthermore, a proper centralized interface between the casing and the traction elements may help to avoid damage to either feature. That is, damage to the casing or a traction element is a likely result where the tractor is not centralized and unbalanced interfaces are present. Such damage may be the result of a sharp edge of the traction element being radially forced against the borehole casing when the tractor is un-centered.
Additionally, centralization of the tractor may be employed as a manner of keeping track of tractor and tool positioning. For example, it may be preferable that a tool of the toolstring arrive at the operation site in a circumferentially centered manner so as to provide a known orientation or positioning of tools relative to the well and one another. This known orientation may be taken advantage of where tools are to interact during the course of operations, for example where one downhole tool may be employed to grab onto and fish out another.
In order to provide centralization as noted above, a centralizer may be associated with the tractor and toolstring. The centralizer may include radially disposed arms biased outwardly from an elongated body of the tractor for contacting sides of the well wall at the borehole casing, thus, centrally positioning the body of the tractor. As described above, tractor arms, and even a toolstring, may also be coupled to the now centralized elongated body, thereby also providing centralization thereto. Thus, tractoring may proceed in a manner optimizing driving friction as detailed above.
Unfortunately, centralization as described above may fail to ensure the optimization of driving friction at the interface of the traction elements and the well in all circumstances. For example, the above described borehole casing may be of a substantially constant circular shape. As such, centralization of the tractor ensures a position of optimized driving friction for the traction elements relative to the well wall. However, in the case of an open-hole well that is lacking a borehole casing an elliptical or other non-circular well shape may be present. In fact, the morphology of the well may change dynamically as the tractor advances therethrough. As a result, problems may arise even where the tractor is initially centralized with the traction elements in a position of optimized driving friction. For example, as the tractor advances through the well, the morphology of the well may change such that the linearly advancing traction elements are no longer in a position of optimized driving friction relative to the well wall. As such, the tractor may fail to advance due to the lack of optimized driving friction and/or damage to the well wall and traction elements may result as described above.